Permanent magnet focusing device



Aug. 26, 1952 R. J. PARKER PERMANENT MAGNET FOCUSING DEVICE Filed July29, 1950 Inventorm Roi Fin J aPKeT;

by A Hi2 AttOWTWGExA Patented Aug. 26, 1952 PERMANENT MAGNET FOCUSINGDEVICE Rollin J. Parker, Schenectady, N. Y., assignor to GeneralElectric Company, a corporation of New York Application July 29, 1950,Serial No. 176,690

1 Claim.

This invention relates to focusing devices and, in particular, to a newand improved permanent magnet focusing device for an electronic tube ofthe cathode ray type.

In a cathode ray tube of the conventional type, either anelectric magnetor a permanent magnet is employed for supplying a magnetic field tofocus an electron beam on a suitable impact target. Such a magneticfield producing device might be referred to as a magnetic lens. Magnetsf the subject type require a means for varying the fiux density of themagnetic field produced within a cathode ray tube to vary the focusingcharacteristics thereof. This variation in flux density may be producedby changing the current passing through an electromagnet Or by changingair gaps in permanent magnets to vary the density of the fiux that actson the electron beam.

The advantage of permanent magnets over electromagnets is the fact thatthe permanent magnet does not require an external power source.Consequently, it is possible with a permanent magnet to use a smallerpower supply in an instrument that employs such a cathode ray tube. Inaddition, a permanent magnet eliminates the necessity of a coil thatwould activate an electromagnet. Furthermore, a permanent magnet wouldeliminate the cost of a variable resistor of some sort that is used inconjunction with an electromagnet to control the amount of currentpassing therethrough to vary the flux density within the cathode raytube.

In the prior art, permanent magnets have been used in conjunction with acentral core that surrounds a cathode ray tube and which is positionedwithin the permanent magnet. The core is axially slidable within thepermanent magnet to decrease the density of the flux that sets up themagnetic field within the cathode ray tube. Such a variation is usuallybrought about by regulating the operative or unshunted axial length ofthe permanent magnet. That is, by sliding the core within the permanentmagnet th effective length of the permanent magnet and as a result thedepth of penetration of the magnetic flux is limited to that portion ofthe magnet which is not separated from the cathode ray tube by the core.With a structure of this sort, however, the flux produced is distributedover a considerable axial length of the cathode ray tube. It isreasonable, and it has been established in the art, that a moredesirable method of producing this magnetic flux is over as short anaxial length of the tube as possible, since the shorter the axialconcentration of the magnetic field, the better the focus produced.Specifically, if the permanent magnet is distributed over a long axialtube length, the flux density that is produced thereby does notconcentrate its effect at a central point and the fringe of a picture onan impact target of a cathode ray tube, consequently, i distorted at theouter edges, even though the picture may be focused properly at thecenter of the tube.

A still further problem that is encountered with permanent magnets ofthe variable efiective axial length type is that when the density of thefiux that is produced within the cathode ray tube is changed, the shapeof the flux path within the tube is also changed. This varying flux pathhas a deleterious effect upon the picture that is produced on the impacttarget of the cathode ray tube in that it produces distortion. It isimportant, then, to produce a permanent magnet wherein the density ofthe flux produced can be varied without changing the shape of the fluxpath within the tube.

It is an object of this invention to provide a new and improvedpermanent magnet focusing device.

It is a further object of this invention to provide a focusing magnetthat operates over a short axial length of the cathode ray tube.

It is a still further object of thi invention to provide a permanentmagnet focusing device that is simple in construction and which is moreeflicient in operation.

It is a still further object of this invention to provide animproved'focusing magnet that maintains a constant shape for the fiuxpath produced thereby while regulating the magnitude of the fiuX.

Further objects and advantages of thi invention will become apparent andthe invention will be more clearly understood from the followingdescription referring to the accompanying drawing, and the features ofnovelty which characterize this invention will be pointed out withparticularity in the claim annexed to and forming a part of thisspecification.

Briefly, this invention comprises the use of an annular Alnicomagnetwith steel lips extending radially from two axially disposed endsthereof. When the lips are brought into or out of engagement by rotatingone relative to the other, the fiux produced by the permanent magnet iseffected by a difierent reluctance and, consequently, this variablereluctance that is offered to the flow of the flux through the internalpath of, the magnet has an effect upon the flux that is produced withinthe cathode ray tube. Specifically, if the lips on the adjacent sectionsof this permanent magnet are aligned, then the least possible resistanceto the flow of flux exists across the lips; consequently, a minimumamount of flux flows through the cathode ray tube. If, on the otherhand, the lips on the adjacent magnetic sections are displacedcircumferentially one from the other, then a great resistance is offeredto the flow of flux through these lips and a maximum amount of fluxflows through the cathode ray tube.

In the drawing, Fig. 1 1s a side view of one of the embodiments of thisinvention';.Fig. 2 isa side view, partly in section, of the new focusingdevice; Fig. 3 is a front view of the focusing device; Fig. 4 is a sideview, partly in section, of variation of the improved focusing device;and Fig. 5 is a front view of the variation.

Referring to the drawing, an annular permanent magnet I made oi Alnicois shown about a cathode ray tube Ia and. in contact with a punching 2held inengagement therewith preferably by magneticattraction. Punching 2is of steel or soft iron comprises a central annular sleeve 3, aradially extending portion 4, and plurality of axiallyextending-portions 5 which are substantially equally spaced about theperiphery of the radially extendingportion' t. An upturned lip 6 ispositioned on each of the axially. extending portions 5. 7 I g r Asecond punching of steel or soft iron comprises a central annularsleet/e8, a radially extending portion 9, and a plurality of axiallyextending portions I8 each having bent up edges I I; The lip portions IIon punching I correspond substantiallyto the lips portions 6 on punching2. Specifically, the lips-'5 and the lips [I are equal in number andtheir shape is such that when axial alignment is given to the centralannular sleeves 3 and 8 the lips I? and'II' are radially equally distantfrom the axial. center of the device.

Punching? is not fixedlysecured to magnet I, but it is held in contacttherewith by magnetic attraction. Punching 1 is provided with agrooved'portiorYIZ havingan internal diameter that equals the externaldiameter of the annular magnet 5., Consequently," when punching I isplaced in an operative position with annular magnet I, the groovedportion I2 surrounds magnet I, to'guide punching I in its rotativemovement relative to punching 2. That is, punching I cannot be movedtransverse to the punching '2 while groove I2 surrounds permanent magnetI.

Sleeves 3 and Sjhave an internal diameter that corresponds substantiallyto the external diameter of the cathode ray tube with which thispermanent magnet is to be used. The advantage of this structure is thatthe annular rings 3 and 8 are in close proximity with the outer shell ofthe cathode ray tube. It is to be noted that the closer these sleevestIandB. are to the cathode ray tube, the greater is the efficiency'ofthe use of the magnetic properties of the annular magnet I.Specifically, if an airgap ispresent between the annular sleeves 3 and 8and the cathode ray tube, then the greater the radialthick-nessof thisair gap, the greater is the reluctance that is offered to the magneticpath of the flux that emanates from the annular magnet I. ihisstructureas shown, gives a greater eiiiciency to the operation of theannular magnet I.

his to befurther noted thatthe' upturned'lips 6 and II of punchings 2and-I, respectively, are sopositioned that: when punching- J is inoperative contact with annular magnet I, a slight air gap 13, say in theorder of .015 inch, exists between the lips 6 and II. The purpose ofthis is to present a minimum air gap between these two portions whileallowing rotative movement of punching 1 relative to punching 2. Withthis structure, and with the minimum air gap between lips 6 and II, asubstantially closed magnetic path is olfered for the flux emanatingfrom annular magnet I. Specifically, the flux can travel from magnet Ito punching I, through axial portion I9 to lip II, across air gap I3into lip 6,

through axially extending portion 5, through punching 2, and back intoannular magnet I.

Under these conditions, since a low reluctance is offered to the now offlux from annular magnet I, a minimum of flux is produced within theinternal diameter of annular magnet I and within thecathode ray tubeabout which this annular magnet would be positioned.

With the design and structure as shown, punching I may be rotatedrelative to punching 2. It is possible to displace lip Ii from lip 6 tovary the dimensions of air gap I3. Specifically, each of the lips t andI I cover a radial sector of about-30.- Fig. 3, for example, shows fourlips G and four lips I I; consequently, there is a total of 8 times 30or 240 of lip portions extending' about the punchings 7 and 2 if thelips do not overlap at all. It is obvious, then, with this structurethat the lips can either be in alignment or completely displaced onefrom the other. When the lips are completely displaced one from theother, a maximum air gap is produced between the lips II and 8';consequently, a high reluctance isofiered to the flow of magnetic fluxfrom annular magnet I through punching I, through axial portion Ill,through lip II into lip 6, because of the great air gap between" lips IIand 6. Under these circumstances, the flux density within the annularmagnet I and within the cathode ray tube is increased. This is what isintended to be accomplished by this new and improved structure-that is,when the lips II and 6' are in alignment, a' minimum amount of flux willpass into the center of the tube about which thismagnet is plac'ed,while, if the lips I! ands are displaced one from the other, a'maximumamount of flux will iiowint'o the center'of the tube.

In an embodiment of this invention shown in Fig. 4, the same typeannular magnet I is shown positioned between a pair of flat parallelcircular discs I5 and I7. Each of these discs has a plurality ofextending ears I8 and I9, respectively, that extend radially beyond theannular magnet vI. The lips I3 and lilaresomewhat akinto-the lips II andtjas shown in Fig. 1. By that'is meant that there are threeears on eachof the discs. It: and ii and each ear occupies approximately a 30sector. The two discs l6 and H are held in parallel juxtaposition by aplurality of rivets or the likelil that are secured to each of the discs16 and I7. Rivets 29 are employed to act asa hearing surface for anannular ring 2 I which has an internal radius that corresponds to thedisplacement of rivets 29 from the center of magnet I. Consequently, thering 2| is revolvable about the bearing rivets '29. Ring 2 I- has amaximum axial length equal tothe distance between the disc I6 and Ii;Ring 2! also has a plurality of aligned indentations 22 whichgin turn,provide a-plurality of lips 23 on'either end of ring 2| whichare ofsubstantially the same circumferential length as the-lips" I8 and -.I 9.Consequently, with this structure, it is possible to have the lips 23 inaxial alignment with the lips l8 and I9, or it is possible to displacethe lips 23 from the lips l8 and I9 and, thus, have the indentedportions 22 in alignment with the lips l8 and I9, whichever happens tobe desired at the particular time. When the lip portions 23 are inalignment with the lips l8 and i9, there is a substantially closedmagnetic circuit from magnet I through lip 19, through lip 23 and ring2|, to an opposite lip 23, to lip 18, and back into magnet I. When lips23 are displaced from lips I8 and I9 and instead the indentations 22 arein alignment with the lips 18 and 19, then a considerable reluctance isoffered to the flow of flux from magnet I through lip l9, across anindentation 22 into ring 2|, and across another indentation 22 into lipI8 and back into the magnet I. Under these conditions, a greater densityof flux flows into the center of the annular magnet I and, consequently,a greater flux density is produced within the center of the cathode raytube.

It is particularly interesting to note that in each of theseembodiments, when it is desired to shunt the flux to decrease the fluxdensity within the tube I, the flux is shunted away from the tube intoupwardly extending lips instead of by decreasing the effective axiallength of the permanent magnet. This is a distinct improvement, sincethe flow of flux into the center of the tube always has the same shapedpath and it is not distorted such as it would be if an axially slidablecore were mounted between the magnet and the cathode ray tube.Consequently, this improved structure acts as a true magnetic lens,since the shape of the flux path is constant, and only the density ofthe flux path is varied. This eliminates the deleterious effect ofhaving a picture with a focused center and a distorted fringe area.

Modifications of this invention will occur to those skilled in the artand it is desired to be understood, therefore, that this invention isnot to be limited to the particular embodiment disclosed, but that theappended claim is meant to cover all the modifications which are withinthe spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

A cathode ray tube permanent magnet focusing device for varying thedensity Of the magnetic field within said cathode ray tube withoutchanging the shape of the flux path comprising an annular permanentmagnet, a pair of soft steel members positioned in parallel planes oneon either end of said magnet, a plurality of supports positionedequidistant from the center of said magnet joining said parallel membersand a soft steel annular ring mounted for rotation on said supportsbetween said parallel members, each of said members having a pluralityof ears extending radially therefrom, said soft steel ring defining aplurality of notches on either axial end thereof. said notches leavingextended portions of said ring corresponding in circumferential extentto said ears on said members and of a width substantially equal to theaxial distance between said members to facilitate the alignment of saidextended portions and said member ears at either end of said ring, saidring being rotatable relative to said ears to increase or decrease thereluctance of a flux path from said permanent magnet through one of saidparallel members through said ring through said other parallel memberinto said permanent magnet, the reluctance of said magnetic path beingat a minimum when the extended portions on said ring are in alignmentwith the corresponding ears on said parallel members and said reluctancebeing at a maximum when the notches on said ring are in alignment withthe ears on said parallel members.

RJOLLIN J. PARKER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,200,039 Nicoll May '7, 19402,212,206 Holst et al Aug. 20, 1940 2,219,193 Mynall Oct. 22,19402,224,933 Schlesinger Dec. 17, 1940 2,234,720 De Tar Mar. 11, 19412,416,687 Fry Mar. 4, 1947 2,418,487 Sproul Apr. 8, 1947 2,431,077 PochNov. 18, 1947 2,433,682 Bradley Dec. 30, 1947 2,442,975 Grundmann June8, 1948 2,501,516 Holden Mar. 21, 1950

